.NET is a general-purpose software development platform, similar to Java. At
its core is a virtual machine that turns intermediate language (IL) into
machine code. High-level language compilers for C#, VB.NET and C++ are
provided to turn source code into IL. C# is a new programming language,
very similar to Java. An extensive class library is included, featuring all the
functionality one might expect from a contempory development platform -
windows GUI development (Windows Form s), database access (ADO.NET),
web development (ASP.NET), web services, XML etc.

2.

What is the CLI? Is it the same as the CLR?

The CLI (Common Language Infrastructure) is the definition of the fundamentals of
the .NET framework - the Common Type System (CTS), metadata, the Virtual
Execution Environment (VES) and its use of intermediate language (IL), and the
support of multiple programming languages via the Common Language Specification
(CLS).

The CLR (Common Language Runtime) is Microsoft's primary implementation of the
CLI. Microsoft also have a shared source implementation known as ROTOR, for
educational purposes, as well as the .NET Compact Framework for mobile devices.
Non-Microsoft CLI implementations include Mono and DotGNU Portable .NET.

3.

What is the CTS, and how does it relate to the CLS?

CTS = Common Type System. This is the full range of types that the .NET runtime understands. Not all .NET languages support all the types in the CTS.

CLS = Common Language Specification. This is a subset of the CTS which all
.NET languages are expected to support. The idea is that any program which
uses CLS-compliant types can interoperate with any .NET program written in
any language. This interop is very fine-grained - for example a VB.NET class
can inherit from a C# class.

4.

What is IL?

IL = Intermediate Language. Also known as MSIL (Microsoft Intermediate
Language) or CIL (Common Intermediate Language). All .NET source code
(of any language) is compiled to IL during development. The IL is then
converted to machine code at the point where the software is installed, or
(more commonly) at run-time by a Just-In-Time (JIT) compiler.

5.

What is C#?

C# is a new language designed by Microsoft to work with the .NET
framework. In their "Introduction to C#" whitepaper, Microsoft describe C#
as follows:
"C# is a simple, modern, object oriented, and type-safe programming
language derived from C and C++. C# (pronounced “C sharp”) is firmly
planted in the C and C++ family tree of languages, and will immediately be
familiar to C and C++ programmers. C# aims to combine the high
productivity of Visual Basic and the raw power of C++."